In a legged mobile robot having a plurality of legs, each being configured by sequentially connecting a plurality of link members from a body through a plurality of joints. For example, in a biped mobile robot having two legs like a human, each leg is configured by sequentially connecting the link members, which correspond to a thigh portion, a crus portion, and a foot portion, through a hip joint, a knee joint, and an ankle joint, respectively, from the body of the robot. In addition, in the legged mobile robot of this kind, each leg's motion for moving the robot is produced by applying driving forces to the joints of each leg by using joint actuators such as electric motors.
Incidentally, in this kind of mobile robot, when, for example, a movement speed thereof is increased, forces acting on the joints of each leg are likely to become relatively large in a foot landing state of each leg (a state of a supporting leg period of each leg), due to a floor reaction force or the like. Consequently, driving forces to be generated to the joint actuators to resist the forces are likely to become relatively large. For example, in a case of allowing the biped mobile robot to run with a gait similar to the gait (a motion pattern of legs) of a running human, the driving force to be generated to the joint actuator of the knee joint becomes large, particularly in the supporting leg period of each leg, according to the knowledge of the inventor and the like of the application concerned. In this case, when the joint actuator is, for example, an electric motor, the aforementioned driving force is generated by a regenerative operation or a powering operation of the electric motor. With any of these operations, however, it is required to energize the electric motor and a power source such as a battery with a high current. Thus, an energy loss owing to Joule heat or the like is likely to be excessively large. Further, since the electric motor with a large capacity is required, the size and weight of the electric motor become large. Note that, in general, the driving forces to be generated to the joint actuators of the knee joint and the like in a foot lifted state of each leg (a state of a free leg period of each leg) are sufficiently smaller than those in the foot landing state of each leg.
Meanwhile, as disclosed in Japanese Patent Laid-Open Publication No. 2001-198864 (especially FIG. 9 of this publication), a biped mobile robot is known in which a spring is provided between two link members (a thigh portion and a crus portion) connected by a knee joint of each leg of the robot.
While moving horizontally, this biped mobile robot transforms energy of horizontal motion of the robot into elastic energy of the spring and stores the elastic energy, thus producing jumping motion of the robot by the use of the elastic energy. In the biped mobile robot having the springs as above, a part of a driving force to be generated to each knee joint is provided by the elastic force of the spring during a part of a running motion period of the robot, particularly during the supporting leg period of each leg. Thus, a burden on the joint actuator of the knee joint can be reduced.
However, in this biped mobile robot, the elastic force of the spring increases in the bending direction of the leg as the bending degree of the leg at the knee joint becomes increasingly large. Further, for example, when the biped mobile robot is moved on a flat floor, the largest bending degree of each leg at the knee joint in the foot lifted state is generally larger than that in the foot landing state of each leg.
Therefore, in the biped mobile robot in the aforementioned publication, the elastic force of the spring is larger during the free leg period than during the supporting leg period, though the driving force to be generated to the knee joint of each leg during the free leg period can be relatively small. Hence, during the free leg period, a driving force for canceling the large elastic force of the spring must be generated to the joint actuator of the knee joint. Thus, the joint actuator of the knee joint has to generate a large driving force during the free leg period of each leg. Consequently, the driving force of the joint actuator cannot be kept relatively small over the entire period of the movement of the biped mobile robot. Moreover, in a case of causing the biped mobile robot to ascend and descend stairs, there occurs a state where the driving force to be generated to the knee joint is relatively small while the bending degree of the leg at the knee joint is relatively large. Therefore, the driving force generated to the joint actuator of the knee joint may end up being large. As a result, it is difficult to improve utilization efficiency of the total energy of the robot.
In the biped mobile robot in the aforementioned publication, the elastic force of the spring increases monotonously as the bending degree of the leg at the knee joint increases. Therefore, when the leg is fully bent during the free leg period and the like of each leg, the amount of elastic deformation of the spring becomes large. Thus, it is required to use a spring which allows such a large amount of elastic deformation, resulting in a large configuration of the spring.
The present invention was accomplished in light of the above-described circumstances. It is an object of the present invention to provide a leg joint assist device to effectively reduce burdens on joint actuators of the legged mobile robot, thereby ensuring favorable energy efficiency, and a reduction in the size of the configuration thereof.